The present invention relates to a mold clamping apparatus and mold opening control method for injection molding machines. More particularly, the present invention relates to a hybrid mold clamping apparatus and mold opening control method which takes advantage of the structure thereof, whereby the mold opening and closing action is electro-mechanically implemented, and the mold clamping action is hydraulically implemented.
As a basic structure for electromechanical mold clamping devices, the mold opening and closing action and the mold clamping action are performed by rotating a ball screw axle using a servo motor, thus moving a movable platen integrally formed with a ball nut which fits onto the ball screw axle. In this type of structure, an extremely large mold-clamping force acts on the ball screw and the ball nut, such that a high-load ball screw is required.
In contrast to this, a mold clamping apparatus has been disclosed in Japanese Patent application H9-29802, in which the mold clamping mechanism is equipped with a mold clamping axle having a trapezoidal screw, angle screw, etc. separate from the ball screw-driven mold opening and closing mechanism, and where the mold clamping force from a hydraulic cylinder operates on the mold clamping axle to clamp a mold. This is a xe2x80x9chybridxe2x80x9d mold clamping apparatus, in which the mold opening and closing mechanism and the mold clamping mechanism are independently constituted. In such a mold clamping apparatus, mold clamping force load will be distributed over an angle screw or trapezoidal screw making surface contact, and no large mold clamping force is loaded on the mold opening and closing ball screw. However, with this type of mold clamping apparatus, the mold clamping axle and the mold clamping nut rub one another during the mold opening and closing operations, whereby frictional resistance is generated between the angle screw, etc. and the nut, whereby drive energy is lost, wear on the screw is created, etc.
Similarly, mold clamping apparatuses into which mold-clamping mechanisms are built around respective multiple tie bars are publicly known through Japanese Utility Model H7-11944 and Japan Patent 2511111 as mold clamping apparatuses in which the mold opening-closing mechanism and the mold clamping mechanism are separately constituted. Such mold clamping mechanisms are equipped with at least a tie bar, a half nut capable of fitting this tie bar, and a mold clamping cylinder which moves the half nut or the tie bar back and forth. When the mold is closed, these half nuts engage the respective tie bar angle screw threads, and the mold clamping cylinder pulls on the tie bar affixed by the half nut to the mold. However, such mold clamping apparatuses have multiple mold clamping mechanisms, and the mold thickness must be adjusted to be equal so that the respective mold clamping mechanisms are equal in order that the respective half nuts equally engage the tie bar angle screw threads during mold clamping. A mold thickness adjustment apparatus which synchronizes and adjusts the mold thickness is therefore required, thus increasing the number of parts in the mold clamping apparatus and requiring more effort for initial assembly adjustment and maintenance of the mold clamping apparatus.
As a partial solution to these problems, the present inventor described in Japanese Patent application H10-108616 for a mold clamping apparatus as shown in FIG. 8.
The mold clamping mechanism comprises a mold opening and closing screw axle 41, which is a ball screw, a mold opening and closing nut 42, which is a ball nut, a mold clamping axle 31 having a movable platen 4, which are attached or integral pieces. Rotational driving of the mold opening and closing screw axle 41 by a servo motor 11 causes the movable platen 4 to advance and retreat to open and close the mold.
The mold clamping mechanism is also equipped with a mold clamping ram 15 contained in a support platen 3, having a half nut device 20 attached at its end. During mold clamping operations, a mold clamping ram 51 receives the mold clamping load over the entire surface of the annular convex portion (annular tooth portion) of the mold clamping axle 31 through the half nut 21 of the closed half nut device 20.
A mold thickness adjustment device 60 comprises a servo motor 61 having a rotary rotary encoder and a brake. A flat gear 65 is rotated by the servo motor causing a cylindrical member 64 to come in and out of contact with the mold clamping ram 51.
A proximity switch 81 is attached to the front end of the half nut device 20. Mold thickness adjustment is performed in order to set and adjust the position of the half nut 21 such that the half nut 21 and the mold clamping axle 31 mesh well when the half nut 21 is tightened at the completion of the mold closing operation.
During the mold thickness adjustment, the servo motor 61 advances the cylindrical member 64. The mold clamping ram 51 which is in contact therewith similarly advances. A proximity switch 81 advances with the half nut device 20. During this forward advance, the proximity switch 81 detects the position at which the half nut 21 and the mold clamping axle 31 mesh well. That position is set in the injection molding machine control device as the mold thickness adjustment position. During subsequent mold opening and closing operations, the cylindrical member 64 limits the advance and retreat positions of the mold clamping ram 51, holding the position of the half nut 21 at the mold thickness adjusted position.
In the above-described mold clamping apparatus, the mold opening and closing mechanism and the mold clamping mechanism are coaxially arrayed compactly on the center axis of the mold clamping device. Movement of the movable platen 4 during mold opening and closing is therefore carried out with a high degree of straightness, and the mold clamping force applied during clamping is accurately applied to the center of the movable platen 4. The half nut 21 is completely open during mold opening and closing operation, so that movement of the movable platen 4 at a high speed and a low frictional resistance is effected without contact between the half nut 21 and the mold clamping axle 31. Furthermore, the mold thickness adjustment device 60 is compactly arranged on the center axis of the mold clamping device, so that there are few mold clamping device parts. This, of course facilitates assembly adjustment, maintenance, and mold thickness adjustment.
However, in the mold thickness adjustment device 60, the retracted mold clamping ram 51 is positioned by mechanical contact with the cylindrical member 64 which is held at the mold thickness adjustment position. Thus, the half nut 21 is held in a position at which it meshes with the mold clamping axle 31. Therefore, the cylindrical member 64 is positioned in the direct vicinity of the rear side of the mold clamping ram 51 at the time of mold clamping. Therefore, if molten resin overfills the die cavity, resulting in an overpack, when the mold clamping ram 51 retracts, it will contact the cylindrical member 64 and be unable to retract further. As a result, as the support platen 3 retreats, a large tension will be imposed on the tie bars 5. In the worst case, this can cause the tie bars 5 to break. It may also be difficult to remove the overpack state as the half nut many not open.
It is therefore an object of the present invention to provide a mold clamping device which has the basic structure of the single mold opening and closing mechanism and single mold clamping mechanism as described in Japanese Patent Application H10-108618, having an improved mold thickness adjustment mechanism whereby accurate positioning and holding of the platen may be achieved using a simple control device which does not rely on mechanical positioning of a half nut device for mold thickness adjustment positioning during mold opening and closing. It is a further object of the present invention to provide a mold clamping device in which overpacking can be easily removed should it occur during injection filling, and where overpacking will not cause an undue strain on the tie bars. It is yet a further object of the present invention to provide a mold clamping device in which assembly, adjustment, and maintenance of the mold thickness adjustment device is easily effected. Yet a further object of the present invention is to provide a mold opening control method where a large mold opening force can be generated as needed at the commencement of the mold opening operation, using a mold clamping device according to the present invention.
An injection molding machine mold clamping apparatus according to the present invention includes a mold clamping apparatus having an axis and a mold opening and closing mechanism having a mold opening and closing screw axle coaxial with said axis. The mechanism includes a ball screw and is axially supported by a support platen. A mold opening and closing nut meshes with the ballscrew. A mold clamping axle attached the mold opening and closing nut, includes, on its outer circumference, square or trapezoidal annular protuberances at a fixed pitch. A movable platen is integrally formed with the mold clamping axle, and a servo motor controls the rotation of the mold opening and closing screw axle. A mold clamping mechanism having a mold clamping ram on the support platen and a half nut device having a half nut which meshes with the annular protuberance in the mold clamping axle is attached to the end of the mold clamping ram. state. Mold clamping is performed by pushing the mold clamping axle, using hydraulic force of the mold clamping ram, mediated by the half nut, with the half nut device in a closed state.
A mold thickness adjustment device, comprises a first detection device which detects the relative position of the mold clamping ram with respect to the support platen and a second detection device, which optically detects the position at which the half nut device engages the mold clamping axle. A mold clamping ram position control device hydraulically controls the holding of the mold clamping ram at a mold thickness adjustment position at which the half nut engages the mold clamping axle.
With this structure, the mold clamping ram position control device hydraulically holds the mold clamping ram at the mold thickness adjustment position during mold opening and closing, and accurately holds the half nut device at the mold thickness adjustment position. Retraction of the mold clamping ram, in case overpacking occurs during injection filling, is allowed, so that overpacking can be easily removed.
The mold thickness adjustment device has a simple structure comprising a first detector, a second detector, and a mold clamping ram position control device; therefore assembly and maintenance of the mold thickness adjustment device may be easily accomplished, and mold thickness adjustment is itself simple.
A mold clamping ram position control device according to the present invention may comprise a switchover valve, which controls switchover of hydraulic fluid supplied to the mold clamping-side oil chamber and the mold opening-side oil chamber on the mold clamping ram, a flow control valve, a pressure control valve, and a pilot check valve. With this structure, the mold clamping ram position control device may take advantage of known mold clamping apparatus hydraulic devices.
The second detection device according to the present invention may comprise an optoelectric detector on the half nut device half nut, wherein a light beam through-hole is formed on the half nut, through which detected light between the optoelectric detector light projection element and a light receiving element can pass.
The detection light beam for the second detection passes through the half nut light beam through-hole, making it unlikely that the second detector will receive external noise. Also, because the second detection device is attached to the half nut directly, it is unlikely that it will protrude forward of the half nut device, eliminating the need of allowing extra length in the mold clamping apparatus.
The mold opening and closing control method according to the present invention is characterized in that it permits selection between two modes: a standard mode in which the mold opening operation is performed by a servo motor, and a strong mode in which the mold opening operation is initially performed by the mold clamping ram.
The mold opening operation is thus selected and controlled, so that when the required mold opening force at the commencement of the mold opening operation is large i.e., the strong mode, the mold clamping ram is caused to retract hydraulically at the commencement of mold opening, such that the half nut device pulls the mold clamping axle, and the mold opening operation is performed using a large mold opening force. The mold opening force is accurately hydraulically controlled, and no large mold opening force load will be distributed over the mold opening and closing mechanism.